The requirements for the signal quality of modulators, for example in transmitting devices, become more stringent as the need for high data rates and increasing mobility grows. The modern mobile radio standards, such as Universal Mobile Telecommunications System (UMTS), Wideband Code Division Multiple Access (WCDMA), Global System for Mobile Communication (GSM), Enhanced Data Rates for GSM Evolution (EDGE), Bluetooth Medium Data Rate or Wireless Local Area Network (WLAN) according to 802.11a/b/g require special modulation types for data transmission which modulate both the phase and the amplitude of a carrier signal at the same time.
Simultaneous amplitude and phase modulation make it possible to achieve higher data transmission rates and thus better bandwidth efficiency. The mobile radio standards mentioned above envisage, for example, the use of quadrature phase shift keying (QPSK), eight phase shift keying (8-PSK), or quadrature amplitude modulation (QAM) as modulation types for the data transmission.
Depending on the selected application for the individual mobile radio standards, these high-quality modulation types are used not only for data transmission from a base station to a mobile communication appliance but also from the mobile communication appliance to the base station.
The modulation types which are used for modern mobile radio standards are particularly sensitive to possible distortion which is produced by various components in a transmission path.
A modulation can be performed using a vector modulation in which data to be transmitted is provided as symbols comprising an in-phase component and a quadrature component. FIG. 8A shows an exemplary constellation diagram of a symbol X which is represented by an in-phase component I and a quadrature component Q. The in-phase component I and the quadrature component Q are modulated using two carrier signals of the same frequency which comprise a phase shift of 90°.
FIG. 8B shows another representation of an exemplary symbol X which is coded with polar coordinates having an amplitude component A and a phase component φ. A carrier signal is phase modulated depending on the phase component φ. Then, an amplitude modulation can be performed with the phase modulated carrier signal depending on the amplitude component A.
Another way of modulating input data is an outphasing modulation. FIG. 8C shows a constellation diagram of an exemplary symbol vector X which is described by two signal vectors S1, S2 having the same amplitude but a different phase φ1, φ2. Compared to the polar modulation, a combination of a phase modulation and an amplitude modulation is replaced by a single phase modulation of each of two carrier signals of the same amplitude. The underlying principle can be explained using the addition theorem:cos(a+b)+cos (a−b)=2 cos(a)cos(b),  (1)withS1=cos(a+b) and S2=cos(a−b).  (2)Assuming that a=ωt+φ(t) and b=arccos(0.5*A(t)), where ω is an angular frequency, φ(t) is a time dependent phase information, and A(t) is a time dependent amplitude information, it results:S1+S2=A(t)cos(ωt+φ(t))  (3)Thus, the phase information φ1 and φ2 of FIG. 8C can easily be derived toφ1=φ(t)+arccos(0.5*A(t)),φ2=φ(t)−arccos(0.5*A(t)).  (4)
FIG. 9 shows an exemplary embodiment of a conventional transmitter arrangement to perform an outphasing modulation. The arrangement comprises a first and a second input 10, 20 to provide a first and a second phase information φ1, φ2. The arrangement further comprises a first loop comprising a first control device CD1 and a first controlled oscillator CO1 to generate a first oscillator signal depending on the first phase information φ1. Accordingly, a second loop comprises a second control device CD2 and a second controlled oscillator CO2 to generate a second oscillator signal depending on the second phase information φ2. The oscillator signals are provided to respective power amplifiers PA1, PA2 which comprise a common control input 350 to control a respective gain factor of the power amplifiers PA1, PA2. The output signals of the controlled oscillators CO1, CO2 correspond to the signal vectors S1, S2 of FIG. 8C.
The outputs of the power amplifiers PA1, PA2 are coupled to a summation element SU1 to combine the amplified oscillator signals. An output of the summation element SU1 is coupled to an antenna ANT.
A high accuracy of the phase modulation in the first and the second controlled oscillator CO1, CO2 can be desirable for certain applications, for example for certain mobile radio standards as GSM/EDGE or UMTS. To receive a desired output signal at the output of the summation element SU1, also the amplitude of the amplified oscillator signals should be the same. Therefore, a phase and/or an amplitude deviation between the two signal paths can be a critical factor for the transmitter arrangement.
To reduce a phase deviation, for example, the combined output signal at the output of the summation element SU1 can be demodulated using an I/Q-demodulator, thus generating respective feedback signals. Depending on a comparison of the fed back and demodulated signals to a desired in-phase and quadrature component, respectively, the generation of the two oscillator signals can be controlled. However, such arrangement needs an additional I/Q-demodulator.